The theoretical framework is a normal mode asymptotic formalism, including both
along branch and across branch coupling,
which provides broadband kernels
for body waveforms and surface waves (NACT).
Ray theory assumes that the sensitivity to structure of body waves is
limited to the infinitesimal ray path and uniform along the ray. On the
other hand, standard surface wave theory ("path average" approximation)
assumes that the sensitivity kernel is 1D, that is, depends only on
the average structure between the source and the receiver. The NACT approach takes into account the finite width
of the sensitivity kernel of a body wave (or higher mode surface waves) around the infinitesimal ray,
and the variation of the sensitivity along the ray.

--- red --->This framework allows the inversion of complete seismogram waveforms, with
accurate 2D sensitivity kernels.
---red --->
The theory used to develop the model is described in :